VMS Help TCPIP Services, Programming Interfaces, Sockets API, gethostbyaddr()  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Searches the hosts database sequentially from the beginning for a
    host record with a given IPv4 address.
    The $QIO equivalent is the IO$_ACPCONTROL function with the
    INETACP_FUNC$C_GETHOSTBYADDR subfunction code.
    Format
      #include  <netdb.h>
      struct hostent *gethostbyaddr  ( const void *addr, size_t len,
                                     int type );

1 - Arguments

 addr
    A pointer to a series of bytes in network order specifying the
    address of the host sought.
 len
    The number of bytes in the address pointed to by the addr
    argument.
 type
    The type of address format being sought. Currently, only AF_INET
    is supported.

2 - Description

    This function finds the first host record in the hosts database
    with the given address.
    The gethostbyaddr() function uses a common static area for its
    return values. This means that subsequent calls to this function
    overwrite previously returned host entries. You must make a copy
    of the host entry if you want to save it.

3 - Return Values

    NULL               Indicates an error; errno is set to one of the
                       following values.
    x                  A pointer to an object having the hostent
                       structure.

4 - Errors

    ENETDOWN           TCP/IP Services was not started.
    HOST_NOT_FOUND     Host is unknown.
    NO_DATA            The server recognized the request and the
                       name, but no address is available for the
                       name. Another type of name server request may
                       be successful.
    NO_RECOVERY        An unexpected server failure occurred. This is
                       a nonrecoverable error.
    TRY_AGAIN          A transient error occurred, for example,
                       the server did not respond. A retry may be
                       successful.